1. Technical Field
The present invention relates to an electrical motor that is driven while its rotational speed is variably controlled.
2. Background Art
Conventionally, rotational speed of an electrical motor is controlled by using an inverter. However, shaft current flows through a rotor shaft of the electrical motor due to a pulse wave of output voltage from the inverter, and thereby discharges occur in a bearing(s) when the shaft current flows to a motor case through the bearing. There is a disadvantage that the bearing may be made electrically corroded by these discharges. In order to prevent such electrical corrosions, it is done to flow electrical charge remained in the rotor shaft to the motor case without flowing it through the bearing by using an earth conductor. Patent Documents 1 (Japanese Granted Patent Publication No. 4485897) and Patent Document 2 (PCT International Application Japanese Translation Publication No. 2010-525787) disclose such an earth conductor. In such an earth conductor, its annular frame is fixed to a motor case, and a rotor shaft is inserted through the annular frame. In addition, many fiber-like conductors are mounted on an inner surface of the annular frame toward the rotor shaft, and distal ends of the fiber-like conductors are contacted with the rotor shaft. According to this configuration, the electrical charge remained in the rotor shaft flows to the motor case through the fiber-like conductors.